1. Field of the Invention
The present invention relates to a body of perovskite material engineered by combining crystallographic engineering with tolerance property relationships for enhancement of piezoelectric properties. The material exhibits a morphotropic phase boundary (MPB) between the rhombohedral and tetragonal phases. More specifically, the present invention relates to the production of a perovskite solid solution having the general formula (1xe2x88x92x)BiMeO3-xPbTiO3, where Me is any suitably sized cation or combination of cations and x is a molar fraction.
2. Description of Related Art
Piezoelectric sensors and actuators have experienced tremendous growth and wide spread application since the initial work with Pb(Ti,Zr)O3, hereinafter xe2x80x9cPZTxe2x80x9d, in the early 1950""s. Since then, PZT-based piezoelectric devices have dominated the world market. They are widely used in underwater sonar, vibration dampening equipment, medical ultrasound transducers, high frequency buzzers and speakers, fuel injection actuators, and precision positioners. Recently there has been interest in incorporating these devices into xe2x80x9cSmart Systemsxe2x80x9d or xe2x80x9cSmart Structures,xe2x80x9d utilizing these device""s dual nature as both a sensor and an actuator. Some proposed applications are for variable control surfaces or noise/vibration suppression in aerospace and automotive applications.
However, the current temperature limitations of PZT-based devices have restricted these types of applications. The PZT-based devices are limited by the Curie temperature (TC) of these materials; the temperature indicating the complete loss of piezoelectric properties, typically less than 350xc2x0 C. It is noted, however, that property degradation typically occurs at temperatures lower than the TC, for example, at approximately xc2xd TC, associated with an irreversible depoling reaction.
The dominance of PZT-based devices has been due to their anomalously high piezoelectric constant values near the MPB between rhombohedral and tetragonal phases. There exist a variety of compositional additions that can be used to tailor the properties of PZT-based piezoelectric materials. These additions result in creation of both xe2x80x9csoftxe2x80x9d and xe2x80x9chardxe2x80x9d piezoelectric ceramics, by either donor or acceptor doping respectively, suitable for a wide range of applications. Research has not, however, provided a way to increase the Curie temperature for PZT-based materials. Additionally, no suitable alternative MPB material systems exhibiting enhanced properties and an increased TC have been developed.
The use of crystallographic engineering in MPB single crystals has been well documented (Paik, D. S., et al., E-field Induced Phase Transition in  less than 001 greater than -oriented Rhombohedral 0.92Pb(Zn1/3Nb2/3)O3-0.08PbTiO3 Crystals. Journal of Applied Physics, 1999. 85(2): p. 1080-1083., Park, S. -E., et al., Crystallographically Engineered BaTiO3 Single Crystals for High Performance Piezoelectrics, 1999, The Pennsylvania State University: University Park., Park, S. -E. and T. R. Shrout, Ultrahigh Strain and Piezoelectric Behavior in Relaxor based Ferroelectric Single Crystals. Journal of Applied Physics, 1997. 82(4): p. 1804-1811.). Crystallographic engineering is a concept to utilize crystal anisotrophy, as well as, an engineered domain configuration to enhance piezoelectric activity. The application of crystallographic engineering in Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-PT) has resulted in single-crystal piezoelectric strain values of 2% compared to the maximum value in PZT ceramics of about 0.15%, as presented in FIG. 1. Property relationships in single crystal materials have linked these high strain values to the tetragonal lattice parameters near the MPB region. The intrinsic strain limit predicted for the PZN-PT system is given by the ratio c/a≈1.02, where c is the c lattice constant and a is the a lattice constant, approaching the measured 2% strain levels in these systems.
Research has also focused on the use of traditional crystal chemistry calculations for predicting and stabilizing new material systems. These calculations are based on the basis geometrical packing of atoms into the idealized perovskite system with the formula ABO3. The calculation used is that of a tolerance factor initially proposed by Goldschmidt and using Shannon and Prewitt""s Ionic Radii (Goldschmidt, V. M., Geochemistry. 1954, Oxford: Clarendon Press and Shannon, R. D., Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta. Cryst., 1976. A(32): p. 751-767.). The tolerance factor for perovskites is given by the formula:   t  =                    r        A            +              r        O                            2            ⁢              xe2x80x83            ⁢              (                              r            B                    +                      r            O                          )            
where rA, rB, and rO are the respective ionic radii, and t is the tolerance factor with t=1 being an ideal perovskite structure. Tolerance factor has been used as a guideline for perovskite phase stability in a number of difficult to prepare material systems (Shrout, T. R. and A. Halliyal, Preparation of Lead-Based Ferroelectric Relaxors for Capacitors. American Ceramics Society Bulletin, 1987. 66(7): p. 704-11.).
Trends in a large number of traditional and bismuth based MPB systems have shown a link between the tolerance factor and Curie temperature. The introduction of the smaller bismuth cation and/or large B-site cation, e.g. Sc, In, Ga, Yb, lowers the tolerance factor, resulting in a significant increase in the Curie temperature of ABO3-PbTiO3 systems, as demonstrated in FIG. 2. A further consequence of an increased Curie temperature is increased tetragonality (increased c/a ratio) as originally observed by Abrahams (Abrahams, S. C., Kurtz, S. K., and Jamieson, P. B., Atomic Displacement Relationships to Curie Temperature and Spontaneous Polarization in Displacive Ferroelectrics, Physical Review, 1968, 172(2): p. 551-3.).
However, tolerance factor relationships have not been combined with crystallographic engineering principals to predict and formulate high temperature perovskite systems, and specifically new morphotropic phase boundary systems.
Accordingly, the perovskites of the present invention were formulated through the use of tolerance factor relationships in combination with crystallographic engineering principals. As a result, the perovskites of the present invention exhibit superior high temperature piezoelectric properties. These high temperature properties result in significant advantages in that the perovskites can be used in high temperature applications previously unavailable due to the lower temperature constraints associated with the prior art perovskites.
The present invention is directed to a new family of high TC MPB systems, based on the perovskite solid solution having the general formula (1xe2x88x92x)BiMeO3-xPbTiO3, where Me is at least one suitably sized cation selected from the group consisting of: scandium, indium, yttrium, ytterbium, other rare earth metals, and combinations thereof, and x is a molar fraction between about 0.50 to 0.90. The perovskite systems of the present invention offer high temperature properties superior to commercially available PZT compositions. The perovskites of this invention exhibit a MPB between the rhombohedral and tetragonal phases, as illustrated in FIG. 3. Further dopant strategies known to those skilled in the art may be used for property optimization of the perovskite systems.
The perovskite of the present invention can be prepared using any method known to those skilled in the art. By way of example, the perovskite crystals can be prepared using a conventional solid state reaction, or whereby the appropriate amount of oxides and/or carbonates, e.g., PbCO3, Bi2O3, TiO2, Sc2O3, etc., are weighed out and intimately mixed. This mixing is followed by calcination, which is the thermal process whereby the interdiffusion of cations and anions takes place to achieve the desired perovskite phase, as determined by x-ray diffraction. The calcined powder is subsequently milled to enhance the powder reactivity and pressed into disks for densification and electrical testing. The polycrystalline perovskite structure is exposed to the minimum electric field (EC) required to achieve maximum polarization (PR), i.e., maximum alignment of the poles of the polycrystalline structure, which results in optimum perovskite properties.
The perovskites of the present invention can be predicted by combining crystallographic engineering principles with basis geometrical packing of atoms in the perovskite structure. From crystallographic engineering, as described above, this increased c/a ratio will potentially result in high strain piezoelectrics, superior to already reported single crystal piezoelectrics. In perovskite ferroelectrics, the spontaneous strain is linked to the transition temperatures. The perovskites of the present invention follow this trend, and owing to their larger transition temperatures, the spontaneous strain and the associated intrinsic dielectric and electromechanical properties also show enhanced piezoelectric properties compared to typical piezoelectrics, such as, PZT. Therefore, by combining the tolerance factor relationship with crystallographic engineering principles, predictions based upon this relationship provide a method for developing new families of high Curie temperature, high strain materials utilizing traditional crystallographic calculations.
One embodiment of the present invention is directed to a perovskite of the formula (1xe2x88x92x)BiScO3-xPbTiO3, where x is between about 0.50 to about 0.90.
A second embodiment of the present invention is directed to a perovskite of the formula (1xe2x88x92x)BiInO3-xPbTiO3, where x is between about 0.50 to about 0.90.
A third embodiment of the present invention is directed to a perovskite of the formula (1xe2x88x92x)BiYbO3-xPbTiO3, where x is between about 0.50 to about 0.90.
All of the perovskites of the present invention exhibit a morphotropic phase boundary between a rhombohedral structure on the bismuth-rich side and a tetragonal structure on the lead-rich side.